Study On Preparation Of Selenium-enriched Candida Utilis And Its Physiological Properties

Selenium, an essential nutritional trace element, is critical to the normal physiology of many mammalian species, including humans. The most important metabolic role of selenium is its function in the active site of selenoenzyme glutathione peroxidase. This enzyme, together with superoxide dismutase and catalase, protects cells against damages caused by free radicals and hydroor lipoperoxides.The deficiency of selenium in the diet is compensated by supplementation in organic or inorganic form. However, Se is both essential and toxic depending on its chemical form and concentration and it has the narrowest tolerance band of any element. It is generally believed that the ingestion of organic Se compounds is better and safer than that contained inorganic Se. But the pure selenomethionine is expensive, and it is necessary to find a low-cost production method of organic seleniumMany organic and inorganic selenium compounds have been investigated as selenium supplements. The Se supplementation using microorganisms has received much attention in recent years. Previous works have shown that yeast was a good carrier for Se biotransformation. Under appropriate conditions, yeasts are capable of producing biomass with high protein content and meanwhile transforming inorganic Se (low bioavailability, potentially toxic) into organic form (safer and highly bioactive), mostly in the form of selenomethionine. Candida utilis, rich in essential amino acids and vitamins, high protein content (520 g/L) and low in fat, has the same nutritional value as fish meal and soybean. As GRAS organism, Candida utilis could be used as food additive and in food and pharmacy industry.Glutathione (GSH), an important physiological active compound, plays significant role in treatment of anti-radiation, tumor, cancer, oxygen toxicity, aging and coordination of endocrine and has no side effects. Therefore, selenium-enriched Candida utilis, with the accumulation of GSH, could be supplement of selenium which could improve the body's immune system and be bound to have a wider market and application prospects.This paper aimed to prepare the selenium-enriched Candida utilis with accumulation of GSH and the study was designed and completed in three aspects as follows:Firstly, the effect of environment conditions on the cell growth and the ability of selenium enrichment with Candida utilis SZU 07-01 was studied. Based on the results of the sole experiment, the L9(34) orthogonal experiment was designed and carried out, and the optimal combination of cultivation conditions in shaking flasks for the preparation of selenium-enriched yeast was achieved as follows: the initial selenium concentration of 10 mg/L, the agitation rate of 200 r/min, the medium volume of 40 mL/500 mL, temperature of 27℃, and the initial pH of 6.0. The cultivation of C. utilis SZU 07-01 was then performed under the optimal conditions, the biomass of yeast reached 11 g/L and the yield of the biotransformation from selenium to organic Se can be obtained up to 90.2%.Secondly, the effect of amino acids addition on the physiological property of selenium- enriched Candida utilis was studied. The results showed that most of amino acids could increase the dry cell weight(DCW),GSH yield(GY),intracellular GSH content(IGC) and recover intracellular redox environment of selenium-enriched Candida utilis. Among of them, amino acids such as 10 mM Asp, 10 mM Val, 8 mM Leu, 10 mM Thr , 10mM Lys and 10 mM Arg could significantly increase DCW by 10-16%; amino acids such as 10 mM Cys, 4 mM Ala, 6 mM Asp, 10 mM Glu and 10 mM Gln could significantly increase glutathione yield by 50-103%; amino acids such as 10 mM Leu, 10 mM Cys, 8 mM Ile and 10 mM Glu could significantly increase IGC by 5-8 times. In addition, amino acids which could increase IGC and the proportion of intracellular distribution of GSH could reduce the intracellular peroxides, and thus maintain the intracellular redox environment.Thirdly, the effect of L-Cys and L-Met on the cell growth,GSH production and accumulation and biotransformation ratio of Se of selenium-enriched Candida utilis was studied by 5 L fermentor. The results showed that there were two useful kinds of strategy of amino acids addition: firstly, addition of L-Met at 0 h and addition of Se at 15 h; secondly, addition of Se at 0 h and addition of L-Cys at 30 h. Addition of L-Cys could increase GSH yield and intracellular GSH content of selenium-enriched Candida utilis more than L-Met while addition of L-Met could increase DCW and ratio of Se biotransformation more than L-Cys. This laid the foundation for industrial production of selenium-enriched Candida utilis with high biomass and more IGC.